Optoelectronics is the term given to the process whereby electricity is inputted to a substance and light is outputted from the substance as a consequence. From the time crystalline silicon was recognized as the dominant material in microelectronics, there has been intensive research to develop light emission from silicon, to enable the development of silicon-based light emitting devices (LEDs) for optoelectronics. Under normal circumstances and conditions, silicon is an indirect band gap semiconductor, unable to emit light efficiently or fast. However, fast emitting photons move at a speed which is about 1000 times that of electrons. Moreover, photons are not affected by magnetic fields, and photon beams can cross one another without mutual interference. These features provide great incentive to develop photonics in place of electronics in many applications. Accordingly, the quest for light emitting silicon-based materials, capable of emitting light efficiently and fast, remains a major research activity.
Canham, L. T. "Silicon Quantum Wire Array Fabrication by Electrochemical Dissolution of Wafers", Appl. Phys. Lett. Vol. 57, pages 1046-1048 (1990) disclosed the efficient room temperature photoluminescence in electrochemically etched silicon or porous silicon. Typically, porous silicon emits light with photon energy twice as large as the band gap energy in crystalline silicon, and the emission can be easily tuned over hundreds of millielectron volts. Electroluminescence in porous silicon has been demonstrated in a variety of devices with an electrolyte, conducting polymer and thin transparent or semitransparent metallic contacts, in a variety of papers.
There are however several potential problems with porous silicon as a source of photoluminescence. Whilst the room temperature photoluminescence quantum efficiency can reach 5%, it degrades in ambient conditions in an hour, mostly caused by high porosity and fragility, unstable hydrogen surface passivation and poor thermal conductivity. The electroluminescence efficiency compared to the photoluminescence efficiency was reported to be more than 100 times less, and the electroluminescence degradation occurs even faster. There are serious problems with the possible integration of porous silicon into standard microelectronic circuitry. Published data indicates that porous silicon is an irregular network of connected silicon nanocrystals with typical sizes of 2-5 nanometers.
Canham, Op. Cit., and Leahmann and Gosele, "Porous Silicon Formation: A Quantum Wire Effect", Appl. Phys. Lett., Vol. 58, pages 856-858 (1990), have suggested that room temperature photoluminescence from porous silicon originates from quantum and spatial confinement effects. The silicon units in porous silicon have a size in the mesoscopic range (2-50 nanometers, nm). Photoluminescence emissions from such unit sizes are in the millisecond--microsecond range, which is too slow for use in LEDs.
It is theoretically predicted that silicon clusters of size 2 nm or smaller (corresponding to clusters of about 20 silicon atoms or smaller) have significantly different electronic properties from larger sized such clusters, and in particular they have much faster photoluminescence, fast enough to be attractive in photoelectronic applications i.e. they are capable of emitting "fast photons".
Yang, Coombs, Sokolov and Ozin, "Nature" Vol. 381, pages 589-592 (Jun. 13, 1996) describe the preparation of oriented mesoporous (pore size 2-5 nanometers, nm, 20-50 .ANG.) silica films grown by surfactant templating at the interface between air and water. The reported films are continuous and are resilient enough to withstand bending, whilst being sufficiently flexible to be transferred onto substrates of different shapes. The films have parallel channels of generally hexagonal cross-section, running predominantly parallel to the surface. They can be prepared under acidic aqueous conditions using carefully controlled mole ratios of water, hydrochloric acid, cationic surfactant and silica source reagent (tetraethylorthosilicate). This mixture is stirred at room temperature and the film is allowed to form under static conditions at the air-liquid interface at 80.degree. C. over a reaction time of minutes to days. The channels are hexagonally close packed with a center-to-center distance of about 5 nm which, allowing for wall thickness, provides channel diameters of about 4 nm. The film forms by a mechanism in which, in the aqueous solution, the silicate building blocks associate with the polar head portion of the surfactant molecules, with the non-polar-hydrophobic tail portions of the surfactant disposed radially inwardly to form a micelle, with a "shell" of silicate building blocks. The silicate building blocks undergo condensation polymerization to form silica, creating hexagonally close packed channels with silica walls.
It is an object of the present invention to provide novel photoluminescent silica-silicon materials capable of fast photon emissions.
It is a further object of the invention to provide methods of preparing such materials.